CSS in JS just isn't required a lot of the time. We have more mature methodologies available.

The web has never lived up to that ideal. Sometimes you need to add a wrapper div with no semantics to make your layout work. Sometimes you need some Javascript to do a special layout CSS doesn't support or work around some other limitation. I've never seen a project that actually had meaningful "separation of concerns" based on technology, whereas almost every componentized project has at least decent separation of concerns along the component axis -- where you can drop a component into a design without thinking about it's internals at all.

> I've never seen a project that actually had meaningful "separation of concerns" based on technology.

With respect, you just need to look a little harder.

I've used react libraries which are fundamentally tied to their styling, due to the fact they use CSS in JS.

Applying custom styling has been more difficult than it should have been.